Embedded cell loopback method and system for testing in ATM networks

ABSTRACT

The present invention relates to a method and apparatus for testing components in ATM networks utilizing loop-back based ATM layer testing. The method and apparatus utilize interfaces and identifier codes to send and loop-back test cells along portions of virtual channels to test the virtual channels.

TECHNICAL FIELD

[0001] The present invention relates to a method and apparatus fortesting components in ATM networks and, amongst other things, to amethod and system for utilizing loop-back based ATM layer testing.

BACKGROUND

[0002] An Asynchronous Transfer Mode (“ATM”) network is made up of anATM switch 5 and ATM endpoints 10. The ATM switch 5 is responsible forcell transit through an ATM network. The job of an ATM switch 5 is toaccept the incoming cells from an ATM endpoint 10 or another ATM switch5. It then reads and updates the cell-header information and quicklyswitches the cell to an output interface toward its destination. An ATMendpoint 10 contains an ATM network interface adapter. Examples of ATMendpoints are workstations, routers, digital service units (DSUs),Remote Terminals, LAN switches, and video coder-decoders (CODECs). FIG.1 illustrates an ATM network made up of ATM switches 5 and ATM endpoints10. Virtual channels are provisioned and dedicated paths that passthrough a number of ATM switches 5 and end points 10 that allow for acircuit switched type connection in a packet-based network. A virtualpath is a logically associated group of virtual channels that passthrough the same components in one or more ATM switches.

[0003] The ATM reference model utilizes the following layers: (i) aphysical layer that is analogous to the physical layer of the OSIreference model and manages the medium-dependent transmission; (ii) anATM layer, which is roughly analogous to the data link layer of the OSIreference model and is responsible for establishing channels and passingcells through the ATM network; and (iii) ATM adaptation layers (AAL)that translates higher layer services into the size and format of an ATMcell. In order to establish and pass through cells, the ATM switch 5translates and routes each cell that it receives. To do this, the ATMswitch 5 uses information in the header of each ATM cell. Theinformation includes, for example: (i) a Virtual Channel Identifier(VCI), which identifies the virtual channel of the cell, and (ii) aVirtual Path Identifier (VPI) that identifies the virtual path of thecell. The ATM switch typically performs this function by utilizing alook-up table.

[0004] Communication to end-users from the high bandwidth backbonenetwork, commonly called the “last mile”, is provided by coaxial cablenetworks, Digital Subscriber Lines (DSL), which are most commonlyAsymmetric Digital Subscriber Lines, twisted pair modem connections, orwireless connections. DSL technology encompasses a variety oftechnologies including but not limited to asymmetric digital subscriberline (ADSL), symmetric digital subscriber line (SDSL) including G.SHDSL,high bit rate digital subscriber line (HDSL), very high bit rate digitalsubscriber line VHDSL, and rate adaptive digital subscriber line (RDSL).In the case of DSL, a central office terminal is typically connected toa high bandwidth network, e.g. a DS1, DS2, DS3, or DS4 network. Thecentral office terminal is coupled to a remote terminal that providesthe DSL connection to the end user. The remote terminals often utilizeminimal processing capabilities in order to reduce their cost.

[0005] In ATM networks most of the monitoring occurs at the physicallayer, meaning that the individual units, e.g. ATM switches, LANswitches, remote terminals, and end units, in the network provide alarmsor other error message when problems with physical transmission aredetermined. However, physical layer monitoring does not monitor thevirtual channels at the ATM layer, specifically the queuing, routing andtranslation functions that provide the virtual channel functionalitythrough different elements in the network. Since ATM and DSL connectionshave extremely high throughputs the inability to test the channel as awhole or in part can result in missing translation or routing problemsthat can result in cells being transmitted along incorrect virtualchannels.

[0006] In addition, ATM utilizes Operations and Maintenance (OAM) layerfunctions to perform testing at the AAL. OAM cells are injected to testeither virtual paths or provisioned virtual channels and can be sentfrom the testing apparatus to an end point or can be looped back betweenany two ATM nodes along the virtual path or virtual channel. However,OAM cells are limited to ATM links in the network and generally requireprocessing capability in each of ATM switches or end units that the OAMcell passes because the cell identity is contained in the cell payloadand these cells cannot be readily distinguished from data cells. Thisincreases the cost of the equipment needed for the end points that inturn drives up the overall cost of the ATM system.

[0007] Therefore, it is desired to have an effective and low cost methodfor testing virtual paths and virtual channels of ATM networks.

SUMMARY

[0008] In one embodiment the present invention is directed toward amethod for determining a status of virtual channels in a distributedcell based network. The method comprises transmitting a test cell from acentral office terminal to a first selected interface along a firstportion of a virtual channel. The method also comprises identifying thetest cell at the first selected interface, transmitting a looped backtest cell from the first predetermined interface along a second portionof the virtual channel, determining if the looped back test cell isreceived at the central office terminal, and determining the status ofthe virtual channel based upon the looped back test cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a block diagram of an ATM network as is known in theart.

[0010]FIG. 2 is a block diagram of an ATM network according to oneembodiment of the present invention.

[0011]FIG. 3 is a flow chart showing a method of virtual channelloop-back testing according to one embodiment of the present invention.

[0012]FIG. 4A is a first loop-back test path according to one embodimentof the present invention.

[0013]FIG. 4B is a second loop-back test path according to oneembodiment of the present invention.

[0014]FIG. 5A is a third loop-back test path according to one embodimentof the present invention.

[0015]FIG. 5B is a fourth loop-back test path according to oneembodiment of the present invention.

[0016]FIG. 6 is an ATM cell according to one embodiment of the presentinvention

DETAILED DESCRIPTION

[0017] Referring to FIG. 2, access network 50 controls the provision ofDSL communication to the end points 55 from the network. The accessnetwork 50 includes a central office terminal 60 and a number of remoteterminals 65 that provide connection to the physical lines over whichthe DSL services are provided to the end points 55. The central officeterminal database 70 maintains information on each of the virtualchannels and virtual paths that are routed through the central officeterminal 60, e.g. whether the virtual channel is “up”, available or“down”, unavailable, and any alarms that are active at any of thephysical components through which the virtual channel passes. Thecentral office terminal processor 75 performs monitoring and control ofthe channel between the central office terminal 60 and the network andcontrols voice connections that are input into the central officeterminal 60. In addition, central office terminal processor 75 injectstest cells into the virtual channels in order to test their status. Asecond central office terminal processor 77 is also provided as aredundant backup for the central office terminal processor 75. However,the second central office terminal processor 77 is only provided forredundancy purposes and is not required for operation according to thepresent invention.

[0018] The central office terminal 60 also includes network interfaces80 and 85 that provide the physical connectivity to different networksthat are provided to the end units. These networks can be voice, ATM, IPor the like. The actual number of network interfaces utilized isdependent on the number of networks that are connected to the centraloffice terminal 60. A number of remote terminal interfaces 90 and 95provide the connection on the end user side of the central officeterminal 60 and to the remote terminals 65. Each of the networkinterfaces 80 and 85 and remote terminal interfaces 90 and 95 have anassociated queue 100, 105, 120 and 125. The queues 100, 105, 120 and 125store cells that are to be transmitted from and that are to be receivedat the interface. Further, it is possible that each queue 100, 105, 120and 125 is divided into separate output queues for each port thatutilizes the interface, in the case where there is more than one portper interface. In addition, central office processor queue 130 queuesthe cells that are transmitted to and from the central office processor75.

[0019] An ATM switch 110 controls the routing and translation for allcells that are received by the central office terminal 60. ATM switch110 preferably has separate ports for each of the interfaces 80, 85, 90,95 and 180 of the central office terminal 60 in order to allow forproper translation and routing of cells. Translation is performed byutilizing a translation table 115 that is part of the ATM switch 110. Ifthere are separate ports for each interfaces 80, 85, 90, 95 and 180 ofthe central office terminal 60, then a separate translation table 115would be required for each of the ports. As used in this description,the term routing will mean both the routing and translation functions ofthe ATM switch 110.

[0020] The remote terminals 65 provide the DSL services to the endpoints 55. In one embodiment, the remote terminals 65 pass through cellswith minimal queuing and translating to reduce the processor power ofthe remote terminal 65. A remote terminal processor 130 controlsoperation of the remote terminal 65. The remote terminal central officeinterface 140 couples the remote terminal to the central officeterminal, while the remote terminal end point interface 145 couples theremote terminal 65 to the end points 55. Each of the remote terminalcentral office interfaces 140 and remote terminal end point interfaces145 has an associated queue 150 and 155 respectively that queues thecells that are received at and transmitted from the interface. The ATMmultiplexer 160 performs the routing and translation functionality forATM cells received and transmitted by the remote terminal 65.

[0021] In order to test the ATM layer of a virtual channel, for examplevirtual channel 165, the central office terminal processor 75 generatesa test cell that is stored in the central office terminal processorqueue 130 for transmission along a test path 170 that comprises aportion of the virtual channel 165 to be tested. The test cell is inputthrough central office processor interface 180 to the ATM switch 110that routes it along the virtual channel 165, to the queue 125associated with the virtual channel 165. The test cell is then read bythe remote terminal interface 90, which determines whether to let thetest cell pass through or to loop it back. The remote terminal interface90 makes this determination based upon an identifier code contained inthe test cell. If the identifier code allows the cell to be passedthrough, it is transmitted to the remote terminal 65 coupled to theremote terminal interface 90. The test cell is received at the remoteterminal central office interface 140 and routed by the ATM multiplexer160 to the queue 155. The remote terminal end point interface 145recognizes the test cell based upon the identifier code contained in thetest cell. Since the cell is a test cell, the remote terminal end pointinterface 145 loops back the cell.

[0022] Once the determination to loop-back the test cell is made, theremote terminal end point interface 145 changes the identifier code ofthe test cell. The looped back test cell is then routed by the ATMmultiplexer 160 and queued in queue 150 for transmission from the remoteterminal 65 to the central office terminal 60. The looped back test cellis transmitted and passes through the remote terminal central officeinterface 140 to the remote terminal interface 90 of the central officeterminal 60, which determines whether to pass the looped back testthrough to the queue 100 or to send it to the central office processorqueue 130 for processing by the central office processor 75 based uponthe changed identifier code in looped back test cell. In the scenariodepicted in FIG. 2 the looped back test cell is transmitted to thecentral office processor queue 130, however it could routed through theATM switch 110 to the queue 100 associated with network interface 80.The network interface 80 would then read the looped back test cell, andbased upon the identifier code, loop the looped back test cell to theATM switch 110 which would route it to the central office terminalprocessor queue 130.

[0023] The test path 170 is determined by the identifier codes of thetest cell. When the central office processor 75 generates a test cell itcan have a code that allows it to pass through every interface exceptfor one. This allows the test path 170 to cover a portion of any and allvirtual channels that include the central office terminal 60. Further,each interface 80, 85, 90, 95, 140 and 145 can be programmed to passthrough or loop back any number of different identifier codes therebyallowing great flexibility as to what portion of the virtual channel thetest path 170 covers. Further, each interface could be programmed sothat it changes identifier codes with specific relationships to receivedidentifier codes so that virtual channels could be tested with one ormore test cells. In addition, it is also possible that an interface,e.g. interface 140, not be programmed to check for identifier codesthereby reducing the processing needed at the interface.

[0024] The central office processor 75 can determine the status of thevirtual channel in several ways. First, if a test cell is not returnedwithin a selected time limit, a problem is determined to exist for thevirtual channel. In one embodiment, test cells are launched one at atime and expected to return before the next test cell is launched. Thepayload is checked for integrity using, for example, a check sumtechnique. In one embodiment, there are fields in the payload to ensurethat the cell received is the expected cell. In addition, the timebetween transmission of the test cell and receipt of the looped backtest cell allows the central office terminal processor 75 to determinecongestion related parameters of the virtual channel. This testingmethod allows for easy detection of cross-talk problems in the ATMswitch 110, ATM multiplexer 160 or any of the queues, since misroutedtest cells are routed to an error queue 185 by the ATM switch 110. Errorqueue 185 is used to collect cells with header errors. The contents ofthe error queue 185 are not usually monitored but the status (number ofcells) is usually monitored.

[0025] Data and management cells transmitted to and from the network andend units 55 are assigned identifier codes that are passed through allof the interfaces and not looped back. In this way, there is nointerference to the data and management cell flow along the virtualchannels.

[0026] Referring to FIG. 3, a test cell is injected from the centraloffice processor 75 along a selected virtual channel, block 200. Thetest cell is interleaved with management and data cells that arereceived from the network for the virtual channel. The test cell thenflows through the virtual channel and is identified as a test cell,directed to a specific port, by an interface for the specific port,block 205. This is done by an identifier code contained in the testcell, and preferably located in the Generic Flow Control (GFC) field ofthe test cell. Once the test cell is identified by the interface, theinterface then changes the identifier code as preprogrammed and the testcell is “looped back” or passed through a selected portion of thevirtual channel to allow a determination as to whether the ATM equipmentis functioning properly at the ATM layer, block 210. The process isrepeated for each interface that is required to loop-back the test cellbased upon changes to the identifier code, block 215. The central officeprocessor then determines if the looped back test cell is receivedwithin a selected time period, block 220. If the looped back test cellis received, then the central office processor analyzes the payload,block 225. If an error is contained in the payload, the statusunavailable is written to the central office terminal database 70 forthat virtual channel and error alert can be transmitted or provided on aview screen associated with the central office terminal 60, block 245.If no error is contained in the payload, then an available status forthat virtual channel is written in the central office terminal database70, block 230.

[0027] If the looped back test cell is not received within a selectedperiod of time, then additional test cells are sent, block 235. If aftera selected number of times the additional test cells are not returned,block 240, an unavailable status is written to the central officeterminal database 70 for the virtual channel and unavailable alert canbe transmitted or provided on a view screen associated with the centraloffice terminal 60, block 245. For a connection between the centraloffice terminal 60 and the remote terminal 65, it should be noted thatif a physical alarm is active on any of the components through which thevirtual channel passes, no error messages need be provided other than tobe written in the central office terminal database 70, since it islikely that the physical component problem is in fact causing the faultfor the virtual channel.

[0028] In order to test each of the virtual channels on a regular basis,in one embodiment, a test cell is sent for each of the virtual channelsthat pass through the central office terminal 60 at selected intervals,e.g., every 60 seconds.

[0029] Referring to FIGS. 4A and 4B, in order to test a fullyprovisioned virtual channel in one embodiment, two test paths areutilized. The test paths are selected so that functions that support thevirtual channel 250 between the network interface 80 and the DSLinterface 145 are tested. In one embodiment, the selected functions arethe routing, translation and queuing functionality for the virtualchannel 250. In FIG. 4A, the first test path 252 includes a test cellwritten by the central office processor 75 and routed by the ATM switch110 to the queue 120 from which it is transmitted by the remote terminalinterface 95 to the remote terminal central office interface 140 of theremote terminal 65. The remote terminal central office interface 140passes the test cell to the ATM multiplexer 160 that routes andtranslates the test cell into the queue 155 of the remote terminal 65.The DSL interface 145 then reads the test cell from the queue 155 andchanges the identifier code in the test cell and writes it to the ATMmultiplexer 160. The test cell is then routed by the ATM multiplexer 160into the queue 150 of the remote terminal 65. The remote terminalcentral office interface 140 then transmits the looped back test cell tothe central office terminal 60.

[0030] At the central office terminal 60, the remote terminal interface95 passes the looped back test cell through the ATM switch 110 and isstored in the queue 100 of the central office terminal 60. The networkinterface 80 of the remote terminal 60 then reads the looped back testcell, changes the identifier code and sends the looped back test cell tothe ATM switch 110 which routes it to central office terminal processorqueue 130, where it is read by the central office processor 75. If thelooped back test cell is not received within a selected time period oran error is detected in the looped back test cell, then an error can bewritten into the central office terminal database 70 and an appropriateerror message or status can be visually displayed or otherwise provided.

[0031] The test path described in FIG. 4A tests all of the queues thatthe virtual channel utilizes. In addition, the test path includes mostof the translation stages that the virtual channel utilizes. However,since the test cell enters the virtual channel from the central officeterminal processor 75 and not the network interface 80 of the centraloffice terminal 60, it is not routed at the input stage from the networkinterface. Because of this, the translation stage from port 80 throughthe switch fabric 110 to the remote terminal queue 120 is not tested. Inother words, a data cell that is received from the network is firstreceived at the network interface 80 and then routed at the associatedport of the ATM switch 110 before being stored in the queue 120. Toremedy this problem, the failure to pass through the port of the ATMswitch 110 associated with the network interface 80, a second test cellas depicted in FIG. 4B is preferably sent a selected time after thefirst test cell as depicted in FIG. 4A.

[0032] Referring to FIG. 4B, a second test cell for transmission along asecond test path 305 is generated by the central office terminalprocessor 75 and is routed by the ATM switch 110 to the queue 100 of thecentral office terminal 60 for transmission to the network. The networkinterface 80 then reads the second test cell, changes the identifiercode of the second test cell and loops it back to the ATM switch 110.ATM switch 110 routes the cell to the queue 120. The looped back secondtest cell is then read by the remote terminal interface 95 andtransmitted to the remote terminal 65. The central office interface 140of the remote terminal 65 then lets the looped back second test cellflow through to ATM multiplexer 160, which routes the looped back secondtest cell to the queue 155. The DSL interface 145 then reads the loopedback second test cell, changes the identifier code and passes the loopedback second test cell to the ATM multiplexer 160 of the remote terminal65, which in turn routes the looped back second test cell to the queue150 of the remote terminal 65. The central office interface 140 of theremote terminal 65 then transmits the looped back second test cell tothe remote terminal interface 95 of central office terminal 60, whichthen changes the identifier code of the looped back second test cell.The looped back second test cell is then passed to the ATM switch 110,which routes the looped back second test cell to the central officeterminal processor queue 130. In this way, the test path in FIG. 4A andthe test path in FIG. 4B pass test cells through each of the functionalqueuing, routing, and translation stages of the virtual channel.

[0033] In one embodiment, test paths in FIGS. 4A and 4B are utilizedsequentially during run time, e.g., when the virtual channel isdetermined to be active. Further, the two test cells need to be spacedapart in time so that the first test cell is to be received, based uponnormal congestion parameters, before the second test cell is to bereceived. In addition, in one embodiment, the virtual channel is testedoften enough, at least once every minute, to ensure a high degree ofreliability. In one embodiment, the entire path of the virtual channelis tested with a single cell rather than the two paths shown in FIGS. 4Aand 4B.

[0034] In FIG. 5A, the third test path 315 includes a test cell writtenby the central office processor 75 that is routed by the ATM switch 110to the queue 120. The queue 120 then changes the identifier code of thetest cell and sends it to the ATM switch 110 which routes the loopedback test cell to queue 100. The network interface 80 then reads thelooped back test cell, changes the identifier code and sends it to ATMswitch 110. The ATM switch 110 then routes the looped back test cell tothe central office terminal processor queue 130 where it is processed bythe central office terminal processor 75. As described with respect tothe first test path in FIG. 4A, the third test path 315 does not includethe routing stage of cells that are received by the network interface80. Therefore, a fourth test path 320 shown in FIG. 5B is provided foruse in conjunction with the third test path 315.

[0035] Referring to FIG. 5B, a fourth test path 320 uses a second testcell that is generated by the central office terminal processor 75 andis routed by the ATM switch 110 to the queue 100 of the central officeterminal 60 for transmission to the network. The network interface 80then reads the second test cell, changes the identifier code of thesecond test cell and loops it back through the ATM switch 110 to theoutput queue 120. The looped back second test cell is then read by theremote terminal interface 95 and looped back to the ATM switch 110,where it is routed to the central office terminal processor queue 130 tobe processed by the central office terminal processor 75. The two testcells need to be spaced apart in time so that the first test cell is tobe received, based upon normal congestion parameters, before the secondtest cell is to be received.

[0036] The test paths depicted in FIGS. 5A and 5B only test the queuing,routing and translation of the ATM switch 110 and central officeterminal 60. The test paths in FIGS. 5A and 5B can be used inconjunction with the test paths in FIGS. 4A and 4B, where a test cell isnot received back at the central office terminal processor. The use ofthe third test path 315 and the fourth test path 320 would provide a wayto isolate where cross-talk or other problems are occurring, by beingable to isolate the problem to either the central office terminal 60 orthe remote terminal 65.

[0037] Test paths 315 and 320 of FIGS. 5A and 5B can be used when novirtual channel is active, either because it is not provisioned or whena remote terminal is not connected to the interface. However, in oneembodiment, a status is written to the central office terminal database70 that indicates that a remote terminal is not connected to theinterface along with any errors or faults for the virtual channel, if infact the remote terminal 65 is not connected.

[0038] In order to allow for operation of the first test path 252,second test path 305, third test path 315 and fourth test path 320, thecentral office terminal 60 is configured to allow a cross connectbetween all of the virtual channels in the central office terminal 60and the central office terminal processor 75. In one embodiment, thetest paths are configured so that a cross connect is set up between thecentral office processor 75 and network interface 80 and DSL interface145 of each remote terminal 65.

[0039] It should be noted that in order for a virtual circuit to bewritten as unavailable, in one embodiment, at least two test cells,e.g., both first and second test cells consecutively, or either thefirst or second test cell within some predefined time period, be lost orhave errors in them. This would prevent the situation of falseunavailable status when the queues for any of the virtual channels arefull due to congestions for short periods of time.

[0040] Referring to FIG. 6, an identifier code 320 is located in the GFCfield 325 of a test cell. The virtual path identifier 330, virtualchannel identifier 335, payload type 340, and congestion loss priority345 of the header 350 need not be utilized in order to test the virtualchannel. Where a data or management cell is transmitted to or from thecentral office terminal 60 or remote terminal 65 they will have anidentifier code 320 of 0h as specified in the International TelephoneUnion specification 1.361, in this case it means that all cells receivedfrom the network and the DSL end points are assigned an identifier codeof 0h. Other identifier codes 320 are checked at the network and DSLinterfaces of the central office terminal 60 and remote terminal 65. Inaddition, the ATM switch 110 will route certain identifier codes 320 tothe central office queue processor queue 130 and the error queue 185.Description of codes for one embodiment and how they are processed whenreceived at the interfaces and ATM switch of the central office terminal60 and remote terminal 65 is depicted in Table I below. TABLE 1 RemoteTerminal GFC Remote Central Field Network ATM Switch Terminal Office DSLCode Interface 80 110 Interface 95 Interface 140 Interface 145 0h PassThrough No effect, cell Pass Pass Pass Through routed according ThroughThrough VPI and VCI 1h Change GFC field No effect, cell Change PassChange GFC to Fh and send to routed according GFC field to Through fieldto Fh and ATM switch VPI and VCI Fh and send send to ATM to ATMmultiplexer switch 2h Change GFC field No effect, cell Change PassChange GFC to Fh and send to routed according GFC field to Through fieldto Fh and ATM switch VPI and VCI Fh and send send to ATM to ATMmultiplexer switch 3h Change GFC field No effect, cell Change PassChange GFC to Fh and send to routed according GFC field to Through fieldto Fh and ATM switch VPI and VCI Fh and send send to ATM to ATMmultiplexer switch 4h Change GFC field No effect, cell Change PassChange GFC to Fh and send to routed according GFC field to Through fieldto Fh and ATM switch VPI and VCI Fh and send send to ATM to ATMmultiplexer switch 5h Change GFC field No effect, cell Change PassChange GFC to Fh and send to routed according GFC field to Through fieldto Fh and ATM switch VPI and VCI Fh and send send to ATM to ATMmultiplexer switch 6h Change GFC field No effect, cell Change PassChange GFC to Fh and send to routed according GFC field to Through fieldto Fh and ATM switch VPI and VCI Fh and send send to ATM to ATMmultiplexer switch 7h Change GFC field No effect, cell Change PassChange GFC to Fh and send to routed according GFC field to Through fieldto Fh and ATM switch VPI and VCI Fh and send send to ATM to ATMmultiplexer switch 8h Change GFC field Route to central Change PassChange GFC to Fh and send to office terminal GFC field to Through fieldto Fh and ATM switch processor queue Fh and send send to ATM to ATMmultiplexer switch 9h Change GFC field No effect, cell Pass Pass ChangeGFC to Fh and send to routed according Through Through field to 8h andATM switch VPI and VCI send to ATM multiplexer Ah Change GFC field Noeffect, cell Change Pass Change GFC to 8h and send to routed accordingGFC field to Through field to Fh and ATM Switch VPI and VCI 8h and sendsend to ATM to ATM multiplexer switch Bh Change GFC field No effect,cell Pass Pass Change GFC to 9h and send to routed according ThroughThrough field to Ah and ATM Switch VPI and VCI send to ATM multiplexerCh Change GFC field No effect, cell Change Pass Change GFC to 8h andsend to routed according GFC field to Through field to Fh and ATM SwitchVPI and VCI 8h and send send to ATM to ATM multiplexer switch Dh ChangeGFC field No effect, cell Pass Pass Change GFC to 9h and send to routedaccording Through Through field to Ch and ATM Switch VPI and VCI send toATM multiplexer Eh Change GFC field No effect, cell Change Pass ChangeGFC to Ch and send to routed according GFC field to Through field to Fhand ATM Switch VPI and VCI 9h and send send to ATM to ATM multiplexerswitch Fh Change GFC field Route cell to Pass Pass Change GFC to 9h andsend to error queue of Through Through field to Fh and ATM SwitchCentral Office send to ATM Terminal multiplexer

[0041] While the embodiments, applications and advantages of the presentinventions have been depicted and described, there are many moreembodiments, applications and advantages possible without deviating fromthe scope of the inventive concepts described and depicted herein. Theinvention should only be restricted in accordance with the scope of theclaims appended hereto and is not restricted by the embodiments,specification or drawings.

What is claimed is:
 1. A method for determining a status of virtualchannels in a cell based network comprising a central office terminalcomprising a plurality of interfaces and an ATM switch, a plurality ofremote terminals each comprising a plurality of interfaces, and aplurality of end points communicating with an interface of the pluralityof remote terminals, the method comprising: transmitting a test cell toa first predetermined interface along a first portion of a virtualchannel including the first predetermined interface, the test cell beinginterleaved with data and management cells transmitted along the virtualchannel; identifying the test cell at the first predetermined interface;transmitting a looped back test cell from the first predeterminedinterface along a second portion of the virtual channel including asecond predetermined interface; determining if the looped back test cellis received at a specified queue of the central office terminal; anddetermining the status of the virtual channel based upon the looped backtest cell.
 2. The method according to claim 1, wherein the test cell andthe loop backed test cell comprise an identifier code, whereinidentifying the test cell at the first predetermined interface comprisesdetermining if the identifier code of the test cell corresponds to aloop-back code for the predetermined interface, and wherein transmittingthe looped back test cell comprises writing a new identifier code intothe test cell prior to transmitting the looped back test cell.
 3. Themethod according to claim 2, wherein the identifier code comprises theGFC field of a cell.
 4. The method according to claim 1, wherein thefirst and second test portions of the virtual channel define a test paththat includes a portion of the virtual channel and wherein the test pathis selected from a group consisting of a first test path that is definedas being from a central office processor interface to an end userinterface of a remote terminal back to a network interface of thecentral office terminal to the central office processor interface, asecond test path that is defined as being from central office processorinterface to a network interface of the central office terminal to theend user interface of the remote terminal back to the remote terminalinterface of the central office terminal to the central office processorinterface, a third test path that is defined as being from a centraloffice processor interface to the remote terminal interface of thecentral office terminal back to the network interface of the centraloffice terminal to the central office processor interface, a fourth testpath that is defined as being from central office processor interface toa network interface of the central office terminal to the remoteterminal interface of the central office terminal back to the centraloffice processor interface.
 5. The method according to claim 1, whereintransmitting a test cell comprises transmitting a test cell on a singlepath to fully test the virtual channel.
 6. The method according to claim4, further comprising selecting the first test path or second test pathwhen the virtual channel is in frame allowing for transmission of cellsbetween the end unit and central office terminal, and selecting thethird or fourth path when the virtual channel is out of frame notallowing for transmission of cells between the end unit and the centraloffice terminal.
 7. The method according to claim 4, wherein aftertransmitting the test cell along the first test path or the second testpath, the method comprises transmitting after a predetermined interval,a second test cell along the other of the first test path or the secondtest path prior to again transmitting the first test cell along a sametest path.
 8. The method according to claim 4, wherein the informationcontained in the looped back test cell comprises information regardingrouting, queuing, looped status and translation of the central officeterminal and the remote terminal along the first test path, second testpath, third test path, or the fourth test path.
 9. The method accordingto claim 1, wherein a plurality of virtual channels are available at thecentral office terminal and wherein the test cell is transmitted foreach virtual channel that is available at the central office terminal atpredetermined intervals.
 10. The method according to claim 1, wherein aconnection between the central office terminal and the remote terminalis a DSL connection.
 11. A central office terminal apparatus comprising:a plurality of interfaces; at least one buffer memory queue selectivelyassociated with the plurality of interfaces, the at least one buffermemory queue adapted to store cells received at and to be transmittedfrom the associated interfaces; an ATM switch in communication with eachof the plurality of interfaces and plurality of buffer memory queues,the ATM switch routing and translating cells received at and to betransmitted from the plurality of interfaces; and a processor incommunication with the ATM switch through a processor interface, whereinthe processor generates test cells that are transmitted from the centraloffice terminal to a designated interface of a remote terminal orcentral office terminal along the paths of selected virtual channelsavailable at the central office terminal, that identifies looped backtest cells transmitted from the designated interfaces, and thatdetermines a status of virtual channel based upon the returned loopedback test cells.
 12. The central office terminal according to claim 111,wherein each test cell comprises an identifier code that indicates thepoint of loop back.
 13. The central office terminal according to claim12, wherein the identifier code comprises the GFC field of a cell. 14.The central office terminal according to claim 111, wherein a channelbetween the remote terminal and the central office terminal is an DSLchannel.
 15. The central office terminal according to claim 111, whereinthe processor generates the test cells at predetermined intervals fortransmission along each of the virtual channels.
 16. The central officeterminal according to claim 11, wherein the processor generates anothertest cell, after the first test cell, transmitted from the processor toanother designated interface along the virtual channel, and wherein thedesignated interface and another designated interface are selected sothat a path comprising the designated interface and the anotherdesignated interface comprise a path between a network input interfaceof the Central office terminal for the virtual channel and an outputinterface of the remote terminal to the end unit along the virtualchannel.
 17. The central office terminal according to claim 11, whereinthe processor transmits cells on the same paths as normal cells for aparticular virtual channel.
 18. A method for determining the status ofvirtual channels provisioned from a central office terminal, the methodcomprising: generating periodic test cells that each comprise anidentifier code, the test cells being transmitted from the centraloffice terminal along test paths that comprise portions of the virtualchannels provisioned at the central office terminal and including remoteterminals; determining if the test cells being transmitted from thecentral office terminal are received as looped back test cells, thedetermination being made based upon a changed identifier code written inthe looped back test cells; and if the looped back test cells arereceived at the central office terminal then determining a status of thevirtual channels by reading a payload of the looped back test cell. 19.A method according to claim 18, wherein the identifier code and thechanged identifier code are located in the GFC field of a cell.
 20. Amethod according to claim 19, further comprising if the looped back testcell is not received at the central office terminal for a virtualchannel then generating a plurality of additional test cells, whichcomprises the identifier code, for transmission from the central officeterminal along the virtual channel, determining if any of the additionaltest cells are received at the central office terminal as looped backadditional test cells, and if none of the additional test cells arereceived at the central office terminal determining that the virtualchannel is unavailable.
 21. The method according to claim 19, furthercomprising if none of the looped back additional test cells are receivedfor a virtual channel, then generating at least one other test cell forthe virtual channel along another test path that includes only elementsof the central office terminal for the virtual channels.
 22. A methodfor determining a status of virtual channels in a cell based networkcomprising a central office terminal and at least one remote terminal,the method comprising: transmitting a test cell to a first selectedinterface of the central office terminal or the at least one remoteterminal along a first portion of a virtual channel including the firstselected interface; identifying the test cell at the first selectedinterface; transmitting a looped back test cell from the first selectedinterface along a second portion of the virtual channel including atleast a second selected interface; determining if the looped back testcell is received at a specified queue of the central office terminal;and determining the status of the virtual channel based upon the loopedback test cell.
 23. The method according to claim 22, wherein the testcell and the loop backed test cell comprise an identifier code, whereinidentifying the test cell at the first selected interface comprisesdetermining if the identifier code of the test cell corresponds to aloop-back code for the selected interface, and wherein transmitting thelooped back test cell comprises writing a new identifier code into thetest cell prior to transmitting the looped back test cell.
 24. Themethod according to claim 23, wherein the identifier code is stored inthe GFC field of a cell.
 25. The method according to claim 22, whereinthe first and second test portions of the virtual channel define a testpath that includes a portion of the virtual channel and wherein the testpath is selected from a group consisting of a first test path that isdefined as being from a central office processor interface to an enduser interface of a remote terminal back to a network interface of thecentral office terminal to the central office processor interface, asecond test path that is defined as being from central office processorinterface to a network interface of the central office terminal to theend user interface of the remote terminal back to the remote terminalinterface of the central office terminal to the central office processorinterface, a third test path that is defined as being from a centraloffice processor interface to the remote terminal interface of thecentral office terminal back to the network interface of the centraloffice terminal to the central office processor interface, a fourth testpath that is defined as being from central office processor interface toa network interface of the central office terminal to the remoteterminal interface of the central office terminal back to the centraloffice processor interface.
 26. The method according to claim 22,wherein transmitting a test cell comprises transmitting a test cell on asingle path to fully test the virtual channel.
 27. The method accordingto claim 25, further comprising selecting the first test path or secondtest path when the virtual channel is in frame allowing for transmissionof cells between the end unit and central office terminal, and selectingthe third or fourth path when the virtual channel is out of frame notallowing for transmission of cells between the end unit and the centraloffice terminal.
 28. The method according to claim 25, wherein theinformation contained in the looped back test cell comprises informationregarding routing, queuing, loop status and translation of the centraloffice terminal and the remote terminal along the first test path,second test path, third test path, or the fourth test path.
 29. Themethod according to claim 22, wherein a plurality of virtual channelsare available at the central office terminal and wherein the test cellis transmitted for each virtual channel that is available at the Centraloffice terminal at predetermined intervals.
 30. The method according toclaim 22, wherein a connection between the central office terminal andthe remote terminal is a DSL connection.
 31. A method for determiningthe status of virtual channels provisioned from a central officeterminal, the method comprising: generating periodic test cells thateach comprise an identifier code, the test cells being transmitted fromthe central office terminal along test paths that comprise portions ofthe virtual channels provisioned at the central office terminal andincluding remote terminals; determining if the test cells beingtransmitted from the central office terminal are received as looped backtest cells; and if the looped back test cells are received at thecentral office terminal then determining a status of the virtualchannels by reading a payload of the looped back test cell.